A varistor is a semiconductor device having a voltagedependent nonlinear resistance. A varistor performs in a manner similar to back-to-back zener diodes in a circuit. When a varistor is exposed to high energy voltage transients, the varistor impedance changes from a very high standby value to a very low conducting value, thus clamping the transient voltage to a safe level. Because of this characteristic, varistors may be used to absorb voltage peaks and protect a load which might be damaged by voltage peaks.
Prior art varistors have served admirably as protection devices. However, if a circuit intended to include a varistor does not in fact include the varistor, or if a varistor is included but the wiring to it is incomplete or has gone open, there will be no protection for the load and there will be no indication that the protective device is missing and/or inoperative. For this reason, a standard two terminal varistor connected in the normal fashion does not constitute a "fail-safe" device.